The present invention relates in general to bicycle front derailleurs of the type comprising a fixed body provided with means for mounting it on a bicycle frame, and a moveable body connected to the fixed body in such a way as to be moveable between two extreme positions in order to shift a bicycle chain from one chainring to another in a series of chainrings of different diameters fitted to the bottom bracket of the bicycle, the said moveable body comprising a forked member consisting of a pair of substantially parallel plates joined at the top by a bridge portion.
The present invention relates in particular to the abovementioned forked member.
FIG. 1 of the accompanying drawings shows a cross section through a forked member 1 of conventional type comprising an inner plate 2 and an outer plate 3 joined at the top by a bridge portion 4. In this description, and in the claims that follow, the expressions xe2x80x9cinner platexe2x80x9d and xe2x80x9couter platexe2x80x9d refer to the usual manner in which the front derailleur is mounted on the bicycle. In the conventional arrangement, the series of chainrings fitted to the bottombracket of the bicycle is on the right-hand side of the bicycle frame, with the larger-diameter chainrings on the outside. Correspondingly, the forked member possesses an inner plate next to the bicycle frame, and an outer plate on the side away from the bicycle, i.e next to the larger-diameter chainrings. FIG. 1 shows how the forked member 1 functions during the shifting of the chain 5 of the bicycle from a smaller-diameter chainring 6 to a larger-diameter chainring 7. During this process the chain is pushed by the inside face 2a of the inner plate 2 of the forked member 1 towards the larger-diameter chainring 7, which is spinning in the direction indicated by arrow A (FIG. 1 shows the chainrings as seen from the front of the bicycle). The combination of the abovementioned movements allows the links of the chain 5 to drop into the teeth of the larger-diameter chainring 7. However, in this final stage the chain 5 may come under such severe twisting that it passes completely over the teeth of the chainring 7 and ends up on the outside of the chainring 7, as illustrated in FIG. 2 (which is a top-down view of the forked member 1 and of the two chainrings 6, 7). Normally this possible problem is contained by correct adjustment of the travel of the derailleur by means of an end stop that determines the position of the inside face 3a of the outer plate 3 of the forked member 1. The adjusted position of the face 3a is determined partly in relation to the type of frame (width and length of the back end of the bicycle, number of sprockets mounted on the rear wheel hub, and the chain line due to the chainring). It should be remembered too that correct adjustment of the derailleur must allow the chain to take up the largest angels required by the number of sprockets fitted to the bicycle without touching the inside faces, as is obvious in FIG. 3 and in FIG. 4 which shows the detail marked IV in FIG. 3 on a larger scale.
In recent times, however, there has been a trend to use ever greater numbers of sprockets on the rear wheel hub, making it necessary to employ chains of reduced width. The adoption of narrower chains has led to the necessity of finding a more efficient solution to the problems explained above.
The object of the present invention is therefore to provide an efficient solution to the abovementioned problems that guarantees highly reliable operation of the derailleur and at the same time has an extremely simple, low-cost structure, leaving it still possible to adapt the derailleur easily and quickly to specific requirements arising from the use of a particular chain.
In order to achieve this object, the subject of the invention is a forked member for a bicycle front derailleur, of the type comprising a pair of substantially parallel plates joined at the top by a bridge portion, characterized in that that plate of the forked member which is to be located next to the larger-diameter chainrings is provided, on its inside face, facing the other plate, with an extra plate that locally reduces the gap between the two plates of the forked member.
The fitting of an extra plate rather than redesigning the derailleur from scratch to satisfy the demands set out above has the advantage that the derailleur can be easily adapted to any further design change by the simple replacement of the abovementioned extra plate.
This extra plate is preferably made of a plastic, preferably with a low coefficient of friction, and is joined by any suitable technique to the plate of the forked member.
In accordance with another preferred characteristic, the abovementioned extra plate has a lower bevel and at least one rear bevel (these terms referring to its position on the bicycle), the first bevel being designed to facilitate the transfer of the chain from the smaller-diameter chainring to the larger-diameter chainring, and the second bevel being designed to allow the chain to take up a highly inclined direction with respect to the central plane of the bicycle when meshing with the outermost sprocket of the hub of the rear wheel of the bicycle.